This application is based on an application No. 11-186614 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a fuel cell system, especially to a system in which electricity is generated by reversing the flow direction of the fluid in the oxidant channel and/or the fuel channel.
(2) Relate Art
Generally, power generation system using the fuel cell reaction (fuel cell system) mainly includes a cell multilayer element. The cell multilayer element is cell units stacked in layers. Each of the cell units includes a cell formed by sandwiching a matrix that is an electrolyte membrane or includes electrolyte (referred to an xe2x80x9celectrolyte membranexe2x80x9d in this specification) between a cathode and an anode. A cell unit is formed by equipping a cell with oxidant channels for supplying the oxidant to the cathode and fuel channels for supplying fuel to the anode. Also, the fuel cell system includes oxidant means for supplying the oxidant to each of the oxidant channels and fuel means for supplying the fuel to each of the fuel channels.
When a proton conductive substance is used as the electrolyte (polymer electrolyte fuel cell, for instance), the concentration of water generated by the fuel cell reaction is especially high downstream from the oxidant. On the other hand, when an oxide ion conductive substance is used (solid-oxide fuel cell, for instance), the concentration is especially high downstream from the fuel. The retention, especially the condensation of the water can block oxidant and fuel channels to lower the reaction downstream from the channels and eventually the reaction of the cell as a whole. This is problematic. One proposed solution to this problem is a fuel cell in a polymer electrolyte fuel cell system (Japanese Patent Laid-Open Publication No. 06-89730). The fuel cell includes condensed water removal means that further includes a non-humidification oxidant gas supplying unit in the midpoint of the oxidant channel for preventing the condensation of water downstream from the oxidant channel and a water absorber so as not to obstacle the gas flow from the gas supplying unit to a part of the oxidant channel upstream from the gas supplying unit. The fuel cell, however, is still problematic. More specifically, the generated water still retains downstream from the non-humidification oxidant gas supplying unit and the construction is complicated. Another proposed solution to this problem is a fuel cell system (Japanese Patent Publication No. 02-21102). In the fuel cell system, the difference of the temperatures around the entrance and the exit of the oxidant channel is detected and the flow direction of the oxidant is reversed when the temperature difference reaches a predetermined value. This is assumed to be effective to solve the retention of the generated water since the water concentration distribution is uniformized by the uniformization of the temperature distribution due to the reverse of the flow direction of the oxidant in the cell.
According to the fuel cell system, however, the flow direction of the oxidant is reversed in the cell multilayer element as a whole, so that the flow of the oxidant is temporarily unstable in the cells when the flow direction is switched at the time of electricity generation. As a result, the output voltage drastically decreases at the time of the switch of the flow direction and the life is shortened.
It is accordingly an object of the present invention to provide a fuel cell system in which the channels are prevented from being blocked by the condensed water while the voltage decrease is prevented as less as possible when the flow directions of the oxidant and fuel are reversed.
The above-mentioned object may be achieved by a fuel cell system that includes: (A) a cell multilayer element that is formed by stacking unit cell structures in layers, a unit cell structure including: a cell that includes a cathode, an anode, and electrolyte sandwiched between the cathode and the anode an oxidant channel unit with a plurality of oxidant channels that cross the cathode and enable oxidant to be supplied to the cathode; and a fuel channel unit with a plurality of fuel channels that cross the anode and enables fuel to be supplied to the anode; (B) an oxidant supplying unit for supplying the oxidant to the oxidant channels; and (C) a fuel supplying unit for supplying the fuel to the fuel channels, wherein the oxidant supplying unit includes a first switching unit for reversing a flow direction of the oxidant in at least one of the oxidant channels and/or the fuel supplying unit includes a second switching unit for reversing a flow direction of the fuel in at least one of the fuel channels.
Here, the fuel cell system, wherein the oxidant supplying unit includes: a first oxidant supplier for supplying the oxidant to a first oxidant channel group that includes at least one of the plurality of oxidant channels; and a second oxidant supplier for supplying the oxidant to a second oxidant channel group that is the plurality of oxidant channels excluding the first oxidant channel group, and the first switching unit independently reverses a direction in which the first oxidant supplier supplies the oxidant to the first oxidant channel group and a direction in which the second oxidant supplier supplies the oxidant to the second oxidant channel group.
Here, the fuel cell system, wherein the first oxidant supplier includes: a first fan; and a first connecting passage for forming an airway between the first fan and the first oxidant channel group, the second oxidant supplier includes: a second fan; and a second connecting passage for connecting the second fan to the second oxidant channel group, and the first switching unit independently reverses a direction of an air current generated by the first fan and a direction of an air current generated by the second fan.
Here, the fuel cell system, wherein the first and second connecting passages are formed by partitioning a space inside of an external manifold with a dividing element.
Here, the fuel cell system, wherein the dividing element is provided with a water-absorbing element.
Here, the fuel cell system, wherein the oxidant supplying unit includes: a first oxidant supplier for supplying the oxidant to a first oxidant channel group that is at least one of the plurality of oxidant channels; and a second oxidant supplier for supplying the oxidant to a second oxidant channel group including all the plurality of oxidant channels that are not in the first oxidant channel group, and the first switching unit reverses the flow direction of the oxidant in at least one of the oxidant channels by changing composition of the first and second oxidant channel groups.
Here, the fuel cell system, wherein the first oxidant supplier includes: a first fan; and a first connecting passage for forming an airway between the first fan and the first oxidant channel group, the second oxidant supplier includes: a second fan; and a second connecting passage for connecting the second fan to the second oxidant channel group, and the first switching unit reverses the flow direction of the oxidant in at least one of the oxidant channels by changing volumes of the first and second connection passages.
Here, the fuel cell system, wherein the first and second connecting passages are formed by partitioning a space inside of an external manifold with a dividing element, and the first switching unit changes the volumes of the first and second connection passages by moving the dividing element.
Here, the fuel cell system, wherein the dividing element is provided with a water-absorbing element.
Here, the fuel cell system, wherein the fuel supplying unit includes: a first fuel supplier for supplying the fuel to a first fuel channel group that includes at least one of the plurality of fuel channels; and a second fuel supplier for supplying the fuel to a second fuel channel group that is the plurality of fuel channels excluding the first fuel channel group, and the second switching unit independently reverses a direction in which the first fuel supplier supplies the fuel to the first fuel channel group and a direction in which the second fuel supplier supplies the fuel to the second fuel channel group.
Here, the fuel cell system, wherein the fuel supplying unit includes: a first fuel supplier for supplying the fuel to a first fuel channel group that is at least one of the plurality of fuel channels; and a second fuel supplier for supplying the fuel to a second fuel channel group that is the plurality of fuel channels excluding the first fuel channel group, and the second switching unit reverses the flow direction of the fuel in at least one of the fuel channels by changing composition of the fuel channels in the first and second fuel channel groups.
Here, the fuel cell system, wherein the directions of the air current generated by the first and second fans are reversed and the dividing element is moved according to one of a cell voltage, a temperature of the oxidant at exits of the oxidant channels, a difference between the oxidant temperature at the exits and a cell representative temperature, and a load current.